There are solar powered power supplies/chargers, so it may be both better (and possibly even cheaper) to get a good reader AND an independent solar charger. More likely they'd each be better than the corresponding elements in a combined device. Plus, a separate charger can be used with >1 device
– DVKDec 31 '13 at 12:56

SolarFocus Technology Company Ltd. has just unveiled one of the most
ingenious and practical solar powered gadgets that we’ve seen in a
long time — a sun-powered cover for the most popular e-reader on the
market, the Amazon Kindle.

A new Color e-Reader is being developed by five media companies based
in Finland. Their intention is to offer a low cost device that can be
charged via solar power and used primarily for the consumption of
newspapers. There is a beta test beginning in November that will see
the Helsingin Sanomat newspaper being delivered by their new cloud
infrastructure.

...

One of the new things this reader has going for it is the solar cell
battery. It will be able to recharge in direct or ambient light and
this is currently the only way you can charge the device.

Here is the company website, but it doesn't seem to be very informative at the moment.

SolarMio provide integrated solar panels into their lighted cases for Kindle 4 and Kindle Touch devices. I bought 2 of their cases for the Kindle 4 in spring 2013 and they seem to be working well in Kenya where they're deployed with Kindles.

BTW: One of the challenges with integrated devices discovered by worldreader http://www.worldreader.org is that people didn't want to leave them to charge in the sun. The risk of them 'disappearing' or being damaged was too great. However, apart from this challenge the SolarMio cases worked well in the field and seem both reliable and effective.